Recently, AV devices and electronic devices such as personal computers of portable or cordless type have been rapidly developed, and secondary batteries which are small in size, light in weight and high in energy density are earnestly demanded as electric sources for driving these devices. Among them, non-aqueous electrolytic solution secondary batteries using a negative electrode containing lithium as an active material are hopefully expected as batteries having high voltage and high energy density.
In the above batteries, lithium-containing metal oxides which show a voltage on the order of 4 V are used for positive electrode active materials, and materials capable of intercalation or deintercalation of lithium, such as carbonaceous materials, are used for negative electrodes.
One of the most important tasks in these non-aqueous electrolytic solution batteries is to ensure the safety.
Particularly, in lithium ion secondary batteries, when they are charged in excess of a given charging voltage due to, for example, troubles of charging control circuits, they are in overcharged state, and lithium ions in the positive electrode are excessively extracted and migrate to negative electrode to cause absorption of lithium in an amount larger than the prescribed design capacity in the negative electrode or to cause precipitation of lithium as metallic lithium on the surface of negative electrode. If the batteries in such a state are further forcedly charged, internal resistance of the batteries increases and generation of heat due to the Joule's heat becomes great to cause abnormal heat generation, and, in the worst case, to result in thermal runaway. By providing a current interrupting switch of temperature sensing type (for example, a positive temperature coefficient thermistor (PTC) or a temperature fuse) outside the batteries, the current is interrupted without fail, and safety can be ensured at the time of generation of abnormal heat. Furthermore, in order to solve the problem of overcharge, a means of interrupting the charging current upon sensing the change of internal pressure of batteries is generally employed as disclosed in U.S. Pat. No. 4,943,497.
However, in the case of using such a mechanical current interrupting mechanism, reduction of cost is difficult and, furthermore, with the batteries becoming smaller and thinner, it becomes structurally difficult to insert the mechanism in the batteries.
For solving the above problems, there is proposed a method of adding to the electrolytic solution an additive which causes a reversible redox reaction, thereby to self-consume the electric energy introduced into the batteries as a redox shuttle (for example, JP-A-1-206571, JP-A-6-338347, JP-A-7-302614, etc.).
However, in the method of using the redox shuttle, when the overcharging current becomes great, there are limits in charge transfer reaction rate and lithium ion transfer rate, and, thus, it cannot be said that the method provides a sufficient safety.
For solving the above problems, JP-9-50822, JP-A-10-50342, JP-9-106835, JP-10-321258, Japanese Patent No. 2939469, and JP-A-2000-58117 propose a means of adding to batteries an aromatic compound having a methoxy group and a halogen group, biphenyl or thiophene, or an aromatic ether compound, which polymerizes at the time of overcharging to result in rising of temperature and, thus, to ensure the safety.